I could not find info via web search on what has changed with contacts from release 5 to release 9 / 11.
I installed WhatsApp on a new Android 11 phone (during switching from Android 5, contacts were copied via Mi Mover app and were seen via preinstalled calling/contacts app) and WhatsApp app could not find any contact using WhatsApp whereas I know some of contacts seen via preinstalled calling/contacts app is using WhatsApp. Investigating that issue I installed 3rd party app (https://f-droid.org/en/packages/com.simplemobiletools.contacts.pro/) to my release 9 other phone. The app did not show contact(s) visible to preinstalled calling/contacts app. I've read history of Android releases and do not recall anything about contacts that result in such behavior. Do newer (than release 5) phones organize contacts differently?
ADDED after comments:
Installed apps were given contacts permissions when asked for it. On 9 Pie phone when installed app added a contact pre-installed one showed it. When I started to add another contact via pre-installed one I was given a choice of com.google.android.contacts and com.simplemobiletools.contacts.pro, I selected com.google.android.contacts, added a contact and again installed com.simplemobiletools.contacts.pro did not show that new contact.
P.S. I recall how nice com.google.android.contacts contacts app was on Android 4, it could show internal and each google account contacts and sync them separately each against each set.
ADDED 2:
Looks like WhatsApp took A LONG (IMHO) time to "replicate" (and does not warn about it!), next day (today) WhatsApp "sees" contacts (I only tried to send messages for a few minutes after install). The question about behavior of other apps mentioned remains. Also on new phone I edited a contact - fine, then on editing some contact pre-installed app asked which account to edit: internal with bluish icon or internal with greenish button (???), I chose bluish cause on opening other one bluish icon was displayed, edited number; upon return to contact it now has both old and new phone numbers and I do not see a way to delete old one...But that IMO could be Xiaomi specifics.
How can I check what app is actually pre-installed for contacts, is it com.google.android.contacts or not?